A high temperature steam electrolyzer (HTSE) is an electrochemical device for generating hydrogen from steam by application of an electric current to a stack of electrolytic cells electrically connected in series and each formed of two electrodes, that is, a cathode and an anode, interposing a solid oxide electrolytic membrane. Generally, steam is introduced at the cathode of each cell powered with electricity, and a reaction of electrochemical reduction of the steam results in the forming of hydrogen on the cathode.
Generally, for a given operating point of the electrolyzer, there exists an electric current to be applied thereto, and the steam or vapor flow to be introduced into the electrolyzer is calculated according to the intensity of the electric current applied to the electrolyzer. Since the current intensity may generally vary from 0 to 100% of the operating range of the electrolyzer, the vapor flow to be generated should also be able to linearly vary from 0 to 100% of the capacity, should only be made of vapor.
Further, an electrolyzer is a system very sensitive to current/gas flow inhomogeneities, such inhomogeneities being indeed capable of causing a premature aging of the electrolyzer. For example, if the vapor flow rate varies around its set point value, an instability of the operating point of the electrolyzer can be observed, resulting in variations of the cell voltage, which is a cause of premature aging. Worse, strong variations of the vapor flow rate result in variations of the pressure by a few tens or hundreds of millibars, which may be sufficient to damage the seals or even crack the electrochemical cells. A vapor flow rate which is as homogeneous and regular as possible is thus desired.
Vapor generation devices generally comprise a heated evaporation surface, having a liquid deposited thereon to generate the evaporation of the liquid. When vapor is generated in a closed chamber, it is necessary to provide a security against overpressures in a device for converting a liquid into vapor operating at constant pressure, particularly at the atmospheric pressure or under a few tens of bars. The security against overpressures enables to limit the risk of explosion of the closed chamber.
In France, the regulations on evaporation devices comprises rules published in the form of a unified technical document (DTU: Document Technique Unifié). Such rules are generally identical in all countries. They are based on the experience of boiler makers and impose the presence of a pressure security element (pressure relief valve or burst disk) installed in an area of the chamber always filled with vapor. Similarly, the rules recommend the installation of pressure measurement equipment (manometer or pressure sensor) in the upper portion of the device.
On conventional boilers containing a liquid water phase in its lower portion and a vapor phase in its upper portion, such a security element or such a pressure measurement equipment should be oversized to operate at the vapor temperature, but they are not necessarily heat-insulated and may form cold spots, which are sources of condensation inside of the device. This is not a problem since the water drops which condensate will fall and mix with the liquid phase already present at the bottom of the device, with no further consequences. This is for example the solution applied in document CN 2,158,515 which integrates a valve in its upper portion.
In the specific case of the system for converting liquid into vapor used in high temperature steam electrolyzers, having as one of its main objects a great pressure stability, it is not acceptable for water drops to condensate on cold spots and to fall at the bottom of the device. They would cause variations of the pressure and of the vapor flow rate due to the nearly instantaneous vaporization of each drop on the very hot floor of the device.
According to an embodiment complying with regulations relative to evaporation devices, it is possible to install a valve and a pressure sensor, making sure to heat-insulate them and to heat them so that they are maintained at a temperature higher than the condensation temperature of the vapor at the considered operating pressure of the device. This requires using a valve and a pressure sensor capable of operating at a temperature in the range from 200 to 300° C., and thus equipment relatively complex to be manufactured and used. The manufacturing and installation cost of such a solution is thus increased.
According to another embodiment complying with regulations relative to evaporation devices, it can be envisaged to install a burst disk and a pressure sensor capable of operating at a temperature in the range from 200 to 300° C., making sure to heat-insulate them. This solution is advantageous from an economical viewpoint, since the burst disk has a low cost. However, the burst disk requires more human action and outages of the vapor device, since it should be replaced each time it has been used. Indeed, it destroys after its operation since it comprises a metal membrane which breaks when the bursting pressure is reached. There thus will be a higher intervention cost than with a valve. Similarly, the pressure sensor should operate between 200 and 300° C., which induces a high complexity and cost.
When complying with regulations relative to evaporation devices, there is not cost-effective solution, that is, only based on low cost components, which allows the pressure measurement and security of a device while limiting the device outage time and reactivation interventions.
The technical problem solved by the embodiments described below is to limit the operating temperature and the maintenance of an evaporation device complying with regulations.